1. Field of the Invention
This invention relates to gas turbines and more particularly to fuel nozzles used with such turbines.
2. Description of the Prior Art
It is desirable for economy purposes to use residual fuels for gas turbines since they are less expensive than distillate fuels. However, residual fuels behave differently than distillate fuels with respect to smoke performance and present particular problems when the turbine must operate within prescribed smoke limits over widely varying loads.
Single shaft gas turbines employed for power generation must operate at constant speed, and thus constant airflow rate, over a widely varying load range. The gas turbines are, therefore, required to operate over a relatively wide range of fuel/air ratios. If the stoichiometry of a combustor is designed for low smoke operation at high load, the combustor must then operate at a very lean primary zone fuel/air ratio at no load. With residual fuel, the primary zone then becomes so lean that the combustion reaction is quenched too early. The temperature does not become high enough, combustion efficiency is low, and the smoke-forming carbon particles are not fully consumed.
This problem of a too lean fuel/air ratio at no load can, of course, be overcome by increasing the fuel/air ratio in the primary zone at no load. However, in that case the primary zone operates in an over-rich condition at higher loads, resulting in an unsatisfactory smoke performance at higher loads.
Various approaches have been taken by the prior art in an attempt to provide satisfactory combustion, and thus smoke performance, with varying fuel flow rates. In one prior art approach two fuel passages are employed in a pressure atomizing nozzle, one having a high pressure drop and the other a low pressure drop. Using the higher pressure drop passage at low fuel flow rates obtains good atomization and combustion efficiency. The lower pressure drop passage opens at increased fuel flow requirements. However, pressure atomizing nozzles are not generally suitable for residual fuels because the high fuel viscosity requires very high fuel nozzle pressures. In particular, the pressure drop at full load when using the high pressure drop passages would be prohibitively high. Air atomizing nozzles, which rely on the interaction of a fuel and air stream to atomize the fuel, have more moderate fuel pumping requirements and so are better suited to use with high viscosity fuels.
In a standard air atomizing nozzle air is used to atomize the fuel. The amount of air employed is customarily independent of the fuel flow rate. At low fuel flow rates the angle of the spray cone is relatively small. As the fuel flow rate increases, the cone "opens up", providing a wider spray angle. As the fuel rate increases further, the downstream end of the cone closes back down.
The present invention provides a means for controlling the spray cone without changing the fuel flow rate so that at any fuel flow rate, control of the spray angle of the cone is provided. The application for which the nozzle of this invention is particularly useful, namely the burning of residual fuel, uses the advantageous characteristics of the nozzle to reduce smoke emission at higher loads in a combustion system which has satisfactory smoke performance at lower loads. It will become apparent as the description of the invention proceeds that it may also be advantageously employed in other combustion systems which have smoke emission problems in, for example, the low or mid-load range. In such other systems control of the spray angle may be effected in a manner appropriate to the requirements of the particular system, using the nozzle structure of the present invention.
The prior art includes a fuel nozzle for gas turbines, shown in U.S. Pat. No. 2,658,800 -- Collinson, for varying the spray angle under different working conditions. Collinson does not discuss what these different working conditions are nor the relationship of the spray angle to particular working conditions. In Collinson's structure the fuel is delivered through an annular orifice and separate air supply passages are provided, one disposed inwardly and the other disposed outwardly of the annular fuel delivery orifice. The air from the two air supply passages impinges on opposite sides of the liquid fuel jet and the spray angle is varied by varying the relative rates of supply of air through the two passages. In the applicants' structure, as will be described in more detail later in the specification, the fuel is delivered through a central passage and both the primary air passage and the secondary air passage are arranged outwardly of the fuel passage. The primary air is employed for initial mixing of fuel and air and establishes a predetermined spray angle at a particular fuel flow rate, such as at low load. The spray angle is then increased by providing secondary air through an annular passage outwardly of the primary air passage in a manner which creates a region of lower pressure substantially at the base of the fuel/air spray. This provides a simpler and substantially more effective arrangement for accurately controlling the spray angle than the nozzle structure of Collinson where the variation of the spray angle is obtained by varying the amount of air impinging on opposite sides of the liquid fuel jet.
The prior art discloses an arrangement, shown in U.S. Pat. No. 3,758,258 -- Kohli, in which the spray angle is increased by creating a low pressure zone near the base of the spray. In Kohli this low pressure zone is created by directing a jet of air outwardly away from the fuel/air spray at a particular angle or by applying suction to a ring surrounding the base of the fuel/air spray. However, the Kohli nozzle operates in a different manner from that of the applicants where, as will be explained in detail later, the low pressure region is created by supplying the secondary air in a swirling manner generally axially of the nozzle rather than outwardly. Moreover, the Kohli disclosure is not directed in any way toward varying the spray angle in accordance with changes in fuel flow rate and thus load so as to obtain consistently good smoke performance under varying conditions, nor is it concerned with controlling the spray angle independently of the fuel flow rate.
In accordance with the present invention, it has been found that improved smoke performance of a gas turbine combustor using residual fuels can be obtained over a wide range of loads by arranging the supply of air to the fuel nozzle in such a manner that the spray angle of the fuel/air mixture may be varied for different loads and may be varied independently of the fuel flow rate. Moreover, this improvement in residual fuel smoke performance is achieved without adversely affecting the smoke performance of the gas turbine when distillate fuels are used.
It is therefore an object of this invention to provide improved smoke performance of gas turbines utilizing residual fuels.
It is another object of this invention to provide improved smoke performance of gas turbines using residual fuels over a wide range of loads from no load to full load.
It is still another object of this invention to provide improved smoke performance of gas turbines using residual fuels without adversely affecting smoke performance when using distillate fuels.
It is a further object of this invention to provide improved ignition capability and improved lean blow-out performance in gas turbines.
It is a further object of this invention to vary the spray angle independently of the fuel flow rate.